(1) Field of the Invention
This invention relates to low-density thermosetting sheet molding compounds (SMC) comprising a treated inorganic clay, a thermosetting resin, a low profile agent, a reinforcing agent, and preferably a low-density filler. The thermosetting SMC are used to prepare exterior and structural thermoset articles, e.g. auto parts, panels, etc.
(2) Description of the Related Art
A molded composite article is a shaped, solid material that results when two or more different materials having their own unique characteristics are combined to create a new material, and the combined properties, for the intended use, are superior to those of the separate starting materials. Typically, the molded composite article is formed by curing a shaped sheet molding compound (SMC), which comprises a fibrous material, e.g. glass fibers, embedded into a polymer matrix. While the mechanical properties of a bundle of fibers are low, the strength of the individual fibers is reinforced by the polymer matrix that acts as an adhesive and binds the fibers together. The bound fibers provide rigidity and impart structural strength to the molded composite article, while the polymeric matrix prevents the fibers from separating when the molded composite article is subjected to environmental stress.
The polymeric matrix of the molded composite article is formed from a thermoplastic or thermosetting resin, which is mixed with fibers used to make a SMC. Thermoplastic polymers “soften” when heated, and recover their plastic properties when cooled. This reversible process can often be repeated many times. The polymers are thermoplastic because they are not chemically cross-linked. Examples of thermoplastic resins include linear polyethylene, polystyrene, acrylic resins, and nylon.
Thermosetting polymers “set” irreversibly by a curing reaction, and do not soften or melt when heated. The reason they do not soften or melt when they are heated is that they chemically cross-link when they are cured. Examples of thermosetting resins include phenolic resins, unsaturated polyester resins, polyurethane-forming resins, and epoxy resins.
Although molded composite article made from SMC based on thermosetting polymers typically have good mechanical properties and surface finish, this is achieved by loading the SMC with high levels of filler. These fillers, however, add weight to the SMC, which is undesirable, particularly when they are used to make automotive or parts of other vehicles that operate on expensive fuels. Therefore, there is an interest in developing SMC that will provide molded composite articles with good mechanical properties that have lower density, in order to improve fuel efficiency.
Additionally, the use of high levels of filler is particularly a problem when highly reactive unsaturated polyesters are used as the thermosetting polymer for making composites. Molded composite articles made from SMC formulations, which employ high reactivity unsaturated polyester resins, often shrink during cure. The shrinkage is controlled with low profile additives (LPA's) and large amounts of fillers, e.g. calcium carbonate, and kaolin clay. Although the resulting molded composite articles have good strength and surface appearance, the density of the composite is high, typically 1.9-2.0 g/cm3. Thus, when used in applications, such as automotive body parts, the added weight lowers fuel efficiency.
U.S. Pat. No. 6,287,992 relates to a thermoset polymer composite comprising an epoxy vinyl ester resin or unsaturated polyester matrix having dispersed therein particles derived from a multi-layered inorganic material, which possesses organophilic properties. The dispersion of the multi-layered inorganic material with organophilic properties in the polymer matrix is such that an increase in the average interlayer spacing of the layered inorganic material occurs to a significant extent, resulting in the formation of a nanocomposite. Although the patent discloses polymer composites, it does not disclose molded composite articles and their mechanical properties, e.g. tensile strength (psi), modulus (ksi), elongation (%), and heat distortion temperature (° C.), nor does it disclose the manufacture of SMC that contains a reinforcing agent, a LPA, and a filler. The problem with using the SMC of the '992 patent is that molded articles prepared with the SMC experience significant shrinkage and are subject to significant internal stress, resulting in the formation of cracks in molded articles.
U.S. Pat. No. 5,585,439 discloses SMC made with an unsaturated polyester resin, and teaches that the mechanical properties of the SMC can be improved if a low profile additive (LPA) is added to the SMC. However, this patent does not teach or suggest the use of nanocomposites in the SMC. The problem with the SMC disclosed in the '439 patent is that when LPA's are used alone, without large amounts of filler (e.g. calcium carbonate and kaolin clay), the molded articles prepared from them have micro and macro voids, which results in molded articles having very low strength. Thus, large amounts of conventional fillers, in addition to LPA's, are required to obtain good strength and surface appearance of molded articles.
All citations referred to under this description of the “Related Art” and in the “Detailed Description of the Invention” are expressly incorporated by reference.